Not a fan of either. Necronomicon inflicts a curse on self which is really dangerous on a squishy class like priest, and tome of pain requires a full wis build to be used effectively, and even then it is still too mana hungry to really do much. If you want a DPS priest I suggest:
Oryx ST set. Merlot is a phenomenal tome for damage, and the rest of the rest really pumps up your DPS stats. My priest out damages my archer with the set, it is very good. You can just use Merlot with other DPS items too
Chaotic Scripture + Mlab robe. Chaotic scripture is pretty mana hungry, but the damage doesn’t wis scale, so you can instead run DPS items and a madlab robe to cut the cost down.
I really want to see necronomicon and tome of pain get buffed. Necronomicon is really cool in that it curses yourself, but it just isn’t good enough to justify using it. I would love to see the damage buffed a lot, along with a lower MP cost.
For tome of pain I would like to see it have more damage and/or a lower MP cost. It is really only useful in midgame dungeons to clear out rooms, trying to use it in harder dungeons is too dangerous. It doesn’t have the damage to kill the enemies, and the healing isn’t good enough to survive until the enemies die.
If you are serious about crafting either one of these, then I say necronomicon as a curse will help out everybody whereas tome of pain just adds a measly ~600 damage on use in a short radius (~900 damage with a radius of ~9 with a full wis set, but you would be doing more damage with a regular DPS set anyway)